This invention relates to the color television art. More specifically it relates to improvements in comb filtering techniques for separating the chroma and liminance information contained in a color television video signal.
In the NTSC color picture transmission system, the total bandwidth of approximately 4.2 MHz is available to transmit color picture information. The picture signal comprises a luminance signal and a chrominance signal. The luminance signal is spread throughout the 4.2 MHz bandwidth at discreet intervals of Fh, the horizontal sweep frequency, with a major portion of the luminance energy contained in a bandwidth less than 2.0 MHz. The chrominance information from the television camera is transmitted by modulating a subcarrier, the subcarrier having a frequency of (N+1/2)Fh, with chrominance information. The sidebands of the subcarrier which contain the chrominance information occur at Fh intervals, but because the subcarrier was selected to be at (N+1/2)Fh, the color subcarrier sidebands, referred to as the chroma signal, are interleaved with the luminance signal frequency components.
One widely used technique for separating the luminance and chroma signals requires a bandwidth limited luminance amplifier in series with a notch filter tuned to the subcarrier frequency. The composite video signal containing chroma and luminance signals is supplied to the bandwidth limited luminance amplifier. The output signal from the amplifier-notch filter combination contains only the low frequency portion of the video signal. This signal contains principally luminance information as the chroma information is found in the high frequency portion of the video signal. The bandwidth limitation of the luminance amplifier results in removal of the chroma signal.
The composite video signal is also supplied to a bandpass filter which is centered at the color subcarrier frequency. The bandpass filter removes most of the luminance signal and yields a signal which contains principally chroma information.
This method of separating the chroma signal from the luminance signal leaves high frequency luminance signals in the chroma channel producing an objectionable interference in the color signal.
The presence of this interference, referred to as cross color in the art, deteriorates picture quality, and the limited bandwidth of the luminance signal limits the picture resolution. Comb filtering techniques have been developed to separate composite chroma and luminance signals which improve upon previous methods of separating chroma and luminance signals. Prior art comb filters, described more particularly in U.S. Pat. No. 3,836,707 make use of a delay line in combination with a summing network to obtain the luminance signal from a composite video signal. Separation occurs when the delay line output is added to the undelayed luminance signal. The response of the comb filter is characterized by a series of bandpass "teeth" having a minimum insertion loss at frequencies of (N)Fh and a maximum insertion loss at frequencies of (N+1/2)Fh. This produces the luminance signal components contained in the composite video signal.
Similarly, the chroma signal may be obtained by subtracting the delayed video siganl from the undelayed video signal. The frequency response of this comb filter is characterized by having a minimum attenuation at frequencies of (N+1/2)Fh and a maximum attenuation at frequencies of (N)Fh.
It is known that the comb filter "teeth" may be narrowed or widened by adding feedback from the filter output to the input of the dealy line. Although narrowing the comb filter teeth improves both signal to noise ratio and cross color rejection of the separated chroma signals, it tends to reduce the vertical definition of the picture produced by the signal. Therefore, most comb filter structures result in a compromise between improved signal to noise ratio and a tolerable loss in vertical definition.